Great Bay Estuary Eelgrass - 1993

Note that yearly variations in collection methods exist. Following is a summary of methods that have been used. The dataset shows the locations where eelgrass was observed in the Great Bay Estuary based on low-altitude aerial surveillance. The study area includes the following waterbodies: Great Bay, the mouth of the Squamscott River, the tidal portion of the Lamprey River, the tidal portion of the Oyster River, the tidal portion of the Bellamy River, the Piscataqua River and Portsmouth Harbor and its associated creeks. Aerial photographs are taken of the study area at 3000 feet at low spring tide with roughly 60% overlap on a calm day without preceding rain events and when the sun is at a low angle to minimize reflection (between 7 am and 10 am). Photographs are taken in late summer, usually late August or early September, depending on tides and weather, to reflect the maximum eelgrass annual biomass. 35 mm film (ASA 200) or a digital camera is used to acquire the images. Typically 300 images are needed to cover the entire Great Bay Estuary. The orientation of the photographs is near-vertical. This is a slight deviation from the NOAA Coastal Change Analysis Program protocol, but follows a published method (Short and Burdick, 1986). The photographs, in the form of 35mm slides or digital computer images, are projected on a screen and the eelgrass images are transferred to a base map. These maps are then digitized using GIS software. The eelgrass habitat mapped from the aerial imagery is verified using the ground truthing data by placing the ground-truthing locations onto the digital image using GIS software. Ground-truthing is done from a small boat during the same season as the photographs are taken. Ground-truth observations are made at low tide. Samples are collected with an eelgrass sampling hook. Positions are determined using GPS. The ground-truth surveys assess 10-20% of the eelgrass beds mapped in the estuary. The aerial survey is completed yearly. The first survey in the series was completed in 1986.需注意，本数据集的采集方法存在年度差异。以下为所采用采集方法的概要说明。本数据集基于低空航空监测结果，记录了大湾河口（Great Bay Estuary）内观测到鳗草（eelgrass）的分布位置。本研究覆盖的水域包括：大湾（Great Bay）、斯夸姆斯科特河（Squamscott River）河口、兰普雷河（Lamprey River）感潮段、奥伊斯特河（Oyster River）感潮段、贝拉米河（Bellamy River）感潮段、皮斯卡塔夸河（Piscataqua River）以及朴次茅斯港（Portsmouth Harbor）及其附属溪流水域。研究区域的航空摄影作业需满足以下要求：飞行高度设定为3000英尺，选择春汛低潮时段开展拍摄，相邻影像重叠率约60%；作业当日需天气晴朗且无前期降雨，太阳角度较低以减少水面反射，拍摄时段限定为上午7时至10时。航空摄影定于夏末进行，通常为8月下旬或9月上旬，具体日期需结合潮汐与天气条件调整，以捕捉鳗草年度生物量峰值时段。图像采集可采用35mm胶片（感光度ASA 200）或数码相机。覆盖整个大湾河口通常需要约300张影像。航空影像采用近垂直拍摄角度。该方式与美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）海岸变化分析项目的规程略有偏差，但遵循已发表的研究方法（Short与Burdick，1986）。采集得到的35mm幻灯片或数字影像将被投影至屏幕，随后将鳗草分布影像转绘至底图之上。随后利用地理信息系统（Geographic Information System, GIS）软件对转绘完成的底图进行数字化处理。通过航空影像解译得到的鳗草生境分布数据，将借助地理信息系统软件将实地核查点位匹配至数字影像中，以此完成精度验证。实地核查作业与航空摄影同期开展，作业载体为小型船舶，且需选择低潮时段进行观测。采样将使用鳗草采样钩完成，点位坐标通过全球定位系统（Global Positioning System, GPS）测定。实地核查覆盖河口内解译得到的鳗草床总面积的10%至20%。本航空监测工作按年度开展，该系列调查的首次作业完成于1986年。